1. Field of the Invention
The invention relates to a line selecting optical arrangement for an excimer laser, and more particularly to an optical arrangement for selecting fewer than all of multiple closely-spaced emission lines of an F2-laser.
2. Discussion of the Related Art
The field of technologies potentially benefitting from the use of excimer lasers is advancing, as is the number and versatility of excimer lasers themselves. An important excimer laser is the F2-laser which emits at wavelengths between 157 nm and 158 nm. Other excimer lasers include such rare gas-halide lasers as XeCl- (308 nm), KrF- (249 nm), ArF- (193 nm), KrCl- (222 nm), and XeF-lasers (350 nm). Several mercury-halides are also used as active gases in excimer lasers, such as HgBr. When the term xe2x80x9cexcimer laserxe2x80x9d is used hereafter in this application, including in the claims, it is meant to refer to any of the lasers mentioned above, and to any other laser which one skilled in the art would label as an excimer laser. Even N2, N2+, CO2 and atomic fluorine may be used as active media within excimer laser discharge chambers. As is apparent, many excimer lasers radiate at ultraviolet wavelengths making them desirable for use as lithography tools.
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems which will be followed by the next generation of ArF-excimer laser systems operating around 193 nm. Vacuum UV (VUV) lithography is expected to use the F2-excimer laser operating around 157 nm.
The output of an F2-laser has been tuned advantageously using prisms inside the laser resonator. See M. Kakehata, E. Hashimoto, F. Kannari, M. Obara, U. Keio Proc. of CLEO-90, 106 (1990). The prism of Kakehata et al. is an extra optical element arranged in the resonator setup along with each of several other components conventionally used with the arrangement.
The output emission spectrum of the F2-laser is characterized by multiple lines including two closely-spaced lines at xcex1=157.629 nm and xcex2=157.523 nm. These two lines are closely spaced. Each line has a linewidth of around 15 pm (0.015 nm). FIG. 1 illustrates these two closely-spaced peaks of the F2-excimer laser spontaneous emission spectrum. The intensity ratio between the two lines is I(xcex1)/I(xcex2)≈7 (See V. N. Ishenko, S. A. Kochubel, and A. M. Razher, Sov. Journ. QE-16, 5 (1986)).
Integrated circuit device technology has entered the submicron regime, thus necessitating very fine photolithographic techniques. The output emission spectrum of the F2-laser including the two lines xcex1 and xcex2 has a bandwidth of at least 106 pm. However, it is desired to use an F2-laser, e.g., for lithography, due to its short wavelength, but a far narrower bandwidth is needed.
Additionally, F2-excimer lasers are characterized by relatively high intracavity losses, due to absorption and scattering in gases and all optical elements. The short wavelength is responsible for these high absorption and scattering losses. Therefore, advisable steps taken to optimize efficiency within the resonator often entail high costs.
It is an object of the invention to provide a radiation source which is useful for material processing at wavelengths around 157 nm, such as may be provided by an F2-laser. It is a further object to provide optics for greatly narrowing the linewidth of the F2-laser, and a method for producing this narrow linewidth. It is a further object to provide an F2-laser with minimal optical losses and a small resonator setup. Such an arrangement would be advantageous in VUV-lithography and VUV-spectroscopy.
The present invention meets the aforementioned objects by providing an optical arrangement which selects one or more, and preferably one, of multiple closely-spaced spectral lines of a laser, particularly an excimer laser, and more particularly an F2-laser. The laser has a discharge chamber containing a laser active gas, an optical resonator and a pair of spaced-apart electrodes for generating a output beam. The characteristic output beam of an F2-laser includes multiple closely-spaced spectral lines in a wavelength range between 157 nm and 158 nm, whereas the F2-laser of the present invention includes an optical arrangement that selects one of these lines. The optical arrangement may also be used for selecting one or more lines of another laser having multiple closely-spaced lines in its characteristic output emission spectrum, thus narrowing its linewidth.
For the F2-laser, the selected line is preferably located at approximately 157.629 nm, and a second line of the multiple closely-spaced spectral lines located at approximately 157.523 nm is not included in the output emission spectrum of the laser, i.e., it is selected out. The wavelength selection optics may include one or more of a birefringent plate, a prism, an etalon and a grating.
If a birefringement plate is used, its thickness is determined by the refractive indices of the plate such that fewer than all of the multiple spectral lines are transmittable through the plate. The plate preferably encloses a gas volume of the laser and thus serves as an opto-mechanical window for the discharge chamber. In a preferred embodiment, a single line of the F2-laser is selected by using a birefringent material to form the discharge chamber window. The window is oriented at Brewster""s angle to the optical path. The preferred material for forming the window is magnesium fluoride (MgF2).
In the field of dye lasers, birefringent Brewster plates have been used for wavelength tuning and line narrowing. See U.S. Pat. No. 3,868,592 to Yarborough et al. In the technique of Yarborough et al., a quartz crystal birefringent Brewster plate is placed in an optical cavity of a dye laser. The birefringent nature of the quartz crystal causes an emission spectrum of the dye laser to be narrowed. Tuning is performed by rotating the plate about an axis normal to the surface of the plate, while maintaining the plate at Brewster""s angle to the optical axis of the dye laser system. Multiple plates are also used wherein each plate has a thickness that is an integral multiple of the thickness of the thinnest plate, for further line narrowing and for spectral distancing of periodically selected lines.
In contrast, in the above aspect of the present invention, a birefringent Brewster window within the resonator closes an excimer laser gas volume obviating the need for an additional optical window, and thereby advantageously reducing optical losses and geometrical size of the excimer laser. Additionally, MgF2 is chosen as the birefringent material for forming the window. MgF2 is advantageously both resistant to fluorine corrosion and transparent to the important UV and Deep UV, particularly at the 157 nm line of the F2-excimer laser.
In an alternative to the preferred arrangement, more than one birefringent plate is aligned along the optical path. Successive plates aligned along the path have optical thicknesses equal to integral values of the thickness of the first plate. Preferably, successive plates have optical lengths twice as great as preceding ones. The alternative arrangement which incorporates this preferred feature has the advantage of greater line narrowing over other configurations.
Another arrangement in accord with the present invention includes a discharge chamber, an optical resonator including an outcoupling unit for partially reflecting incident light back along an optical axis, and a laser active volume within a housing for emitting a broad spectral band of light, wherein the laser active gas fills the discharge chamber. The arrangement further includes a wavelength selection unit within the housing, and preferably within the discharge chamber. In this way, line selection is performed in a manner which optimizes the combination of optical and discharge efficiency, resonator size and costs. The wavelength selection unit is preferably a prism, but may also be a grating, an etalon, a birefringent plate, or another wavelength selecting optical device.